For the "giant" breeds (not just "large"), the lower the calcium the better during the growth period. As Hiraeth said, under 1.5% for calcium, balanced with a little less phosphorus.
Many mastiff lines also are sensitive to both grains and chicken in kibble. You can ask the breeder if they know of any chicken issues with their pups, or you can just opt to start with a chicken & grain-free food to begin with.
We started out with Solid Gold Wolf Cub, but it's probably a mid-grade food. Acana, Orijen, Fromm and many others are on the "good" list - as long as you pick a recipe at the lower calcium range.
DogFoodAdvisor-dot-com is a wealth of information on different foods.
If you like to read and do research, here's a link to some of their discussions on feeding big puppies:
http://www.dogfoodadvisor.com/forums/topic/large-and-giant-breed-puppy-nutrition/
Puppy food has more fat and calories and is often formulated to promote FAST growth... which is BAD for big bones. You want the muscle to grow at pace with the bones, and the slower the better. So an adult or all-life-stages food is often recommended for giant breeds.
You might also research raw feeding, if you have resources to prepare your own meals (i.e. hunters can really make it cost-effective). We converted our pup at 6 months to a "whole prey model raw" diet (a google search returns LOTS of help), and I love giving her fresh food (and I think she likes it too, HA!).
Congratulations on your new additions!